Sulphur containing Amino acid is?

Appeared in: NIA - 29 January 2023

Explanation

Methionine is classified as a sulfur-containing amino acid because its side chain features a thioether group (-S-CH₃).
Along with Cysteine, it is one of the two proteinogenic amino acids that contain sulfur.
Methionine is also an essential amino acid, which means it cannot be synthesized by the human body and must be obtained through diet.

Option A: Phenylalanine is an aromatic amino acid. Its side chain contains a bulky, nonpolar phenyl group, not a sulfur atom.
Option B: Glycine is the simplest amino acid. Its side chain is just a single hydrogen atom, so it does not contain sulfur.
Option D: Valine is a branched-chain amino acid (BCAA). Its side chain is a nonpolar isopropyl group and lacks a sulfur atom.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Classification of Amino Acids as background academic context rather than a clinical decision trigger.
Methionine is an essential amino acid, meaning its dietary intake is vital for protein synthesis, tissue repair, and overall health. Deficiencies can impair growth and protein production.
Methionine is the precursor to S-adenosylmethionine (SAM), the body's universal methyl donor. SAM is critical for numerous metabolic reactions, including DNA methylation, neurotransmitter synthesis, and detoxification.
What if? If a patient has homocystinuria, a genetic disorder of methionine metabolism, they cannot properly break down methionine. This leads to a toxic buildup of homocysteine, requiring a specialized diet low in methionine to prevent complications like blood clots, skeletal abnormalities, and intellectual disability.

How to Approach the Question

First, identify the key characteristic the question is asking for: a 'Sulphur containing' amino acid.
Next, recall the basic classification of the 20 standard amino acids based on their side chains (R-groups). The main categories include aliphatic, aromatic, acidic, basic, and sulfur-containing.
Systematically evaluate each option against this classification.
Analyze Phenylalanine: recognize it as aromatic due to its benzene ring.
Analyze Glycine: recall it is the simplest amino acid with just a hydrogen side chain.
Analyze Methionine: remember it is one of the two common sulfur-containing amino acids (the other being Cysteine).

Concept Tested & Keywords

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E6 Biochemistry U Satyanarayana— Part 1 (pp 26-420 of 840) pp. 36-38, 34-36

Practise the full NIA - 29 January 2023

Attempt every question from this paper in a timed mock, then review the full solution for each one.